feat(backend): harden draft, pricing and publish contracts

- unify typed API errors across draft, pricing and publish flows
- add stale draft and publish-state mutation guards
- add publish readiness contract and guarded publish flow
- add sellability reason codes to test seat preview
- add pricing diagnostics and strengthen snapshot/publish lifecycle consistency

backend/app/services/api_errors.py

@@ -3,12 +3,17 @@ from __future__ import annotations
 from fastapi import HTTPException, status
 
 
-def raise_conflict(*, code: str, message: str, details: dict | None = None) -> None:
+def raise_conflict(
+    *,
+    code: str,
+    message: str,
+    details: dict | None = None,
+) -> None:
     payload: dict = {
         "code": code,
         "message": message,
     }
-    if details:
+    if details is not None:
         payload["details"] = details
 
     raise HTTPException(
@@ -17,13 +22,18 @@ def raise_conflict(*, code: str, message: str, details: dict | None = None) -> N
     )
 
 
-def raise_unprocessable(*, code: str, message: str, details: dict | None = None) -> None:
+def raise_unprocessable(
+    *,
+    code: str,
+    message: str,
+    details: dict | None = None,
+) -> None:
     payload: dict = {
         "code": code,
         "message": message,
     }
-    if details:
-        payload["details"] = details
+    if details is not None:
+        payload.update(details)
 
     raise HTTPException(
         status_code=status.HTTP_422_UNPROCESSABLE_ENTITY,